LED drivers can be classified into isolated type and non-isolated type. The LED driver of isolated type needs a transformer to separate the primary side and the secondary side and thus needs higher costs. Differently, the LED driver of non-isolated type does not need a transformer, so the costs thereof are lower. However, the LED driver of non-isolated type easily triggers an abnormal or an unstable state occurring at the circuit caused by an instant high voltage variation.
FIG. 1 shows a conventional non-isolated linear LED driver 10, which includes a bridge rectifier 12 for rectifying an AC voltage Vac to generate a DC voltage VIN for LEDs, and an integrated circuit (IC) 14 for controlling the LEDs to be lighted. In the IC 14, switches 18, 20, 22, and 24 are serially connected to the LEDs via pins S1, S2, S3, and S4, respectively, and a controller 16 controls the switching of the switches 18, 20, 22, and 24 to decide which LED is to be lighted. The linear LED driver 10 may experience an instant high voltage variation caused by a lightning stroke, a system electro-static discharge (ESD), AC in multiple touch, or a triode alternating current (TRIAC) dimming.
Taking the TRIAC dimming as an example shown in FIG. 2, a conventional TRIAC dimmer includes resistors R1 and R2, a capacitor C1, a bidirectional trigger diode (DIAC) 26, and a TRIAC switch 28. The resistor R1 is a variable resistor. The TRIAC switch 28 is off at the beginning and consequently the AC voltage Vac is not inputted to the load. The resistors R1 and R2 generate a current according to the AC voltage to charge the capacitor C1. When the voltage at the capacitor C1 reaches a breakover voltage of the DIAC 26, the DIAC 26 will be turned on so as to turn on the TRIAC switch 28. When the TRIAC switch 28 is turned on, the AC voltage Vac is inputted to the load and the capacitor C1 starts discharging. The TRIAC switch 28 keeps in the on state until the AC voltage Vac becomes zero or until a current I1 passing through the TRIAC switch 28 is lower than a threshold. That is to say, the TRIAC dimmer turns the AC voltage Vac into an AC phase-cut voltage that includes a conduction angle. The AC phase-cut voltage Vtr will be rectified by the bridge rectifier 12 in FIG. 1 to generate the DC voltage VIN as shown by a waveform 30 in FIG. 2. As shown by the waveform 30 in FIG. 2, the DC voltage VIN generated by the TRIAC dimming will instantly jump to a high voltage from 0V, which causes an instant high voltage variation.
FIG. 3 shows the switch 18 in FIG. 1. The AC voltage Vac is a high voltage, so the switch 18 has to be a high-voltage component. Generally, the switch 18 can be a metal-oxide-semiconductor field effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT). FIG. 4 shows waveforms of the DC voltage VIN that is subjected to an instant high voltage variation, in which the waveform 32 represents the voltage of the pin S1, and the waveform 34 represents the voltage of a control terminal of the switch 18. Referring to FIGS. 1, 3, and 4, an input terminal 182 of the switch 18 is coupled to the pin S1, a control terminal 184 of the switch 18 is coupled to the controller 16, and an output terminal 186 of the switch 18 is coupled to a ground terminal. When the input voltage VIN occurs an instant high voltage variation, the voltage at the pin S1 raises rapidly, as shown by the waveform 32 in FIG. 4, thereby generating a large current to charge a parasitic capacitance Cdg1 between the input terminal and the control terminal of the switch 18. Consequently, the voltage of the control terminal of the switch 18 raises rapidly, as shown by the waveform 34 in FIG. 4. When the voltage of the control terminal of the switch 18 is higher than a threshold Vth, an unstable state will be resulted. The switch 18 can be even burned out. In some applications, the output terminal 186 of the switch 18 may be coupled to some low-voltage circuits. When the voltage at the pin S1 raises rapidly, a large current will go through the switch 18, which incurs the voltage of the output terminal 186 of the switch 18 raises rapidly. As a result, the low-voltage circuit connected to the output terminal 186 of the switch 18 cannot endure the instant high voltage variation, and therefore is burned out.
U.S. Patent Publication No. 2010/0253245 discloses a method for solving the instant high voltage variation, which inserts a current limiting circuit like an overvoltage protection circuit between the LED driver and the LEDs. The current limiting circuit detects a voltage on the LEDs to control a switch that is serially connected to the LEDs. However, the current limiting circuit needs a large component that has to be additionally installed out of the IC. Therefore, it introduces a large parasitic capacitor, which incurs a slower response of the current limiting circuit. Moreover, U.S. Patent Publication No. 2010/0253245 merely solves the problem of the instant high voltage variation that is caused by surge. The instant high voltage variation that is caused by the system ESD, AC in multiple touches, or the TRIAC dimming, is still not improved.